Femtosecond Laser Surface Processing (FLSP) is a versatile technique for the fabrication of a wide variety of micro/nanostructured surfaces with tailored physical and chemical properties. Through control over processing conditions such as laser fluence, incident pulse count, polarization, and incident angle, the size and density of both micrometer and nanometer-scale surface features can be tailored. Furthermore, the composition and pressure of the environment both during and after laser processing have a substantial impact on the final surface chemistry of the target material. FLSP is therefore a powerful tool for optimizing interfacial phenomena such as wetting, wicking, and phasetransitions associated with a vapor/liquid/solid interface. In the present study, we utilize a series of multiscale FLSPgenerated surfaces to improve the efficiency of vapor generation on a structured surface. Specifically, we demonstrate that FLSP of stainless steel 316 electrode surfaces in an alkaline electrolysis cell results in increased efficiency of the water-splitting reaction used to generate hydrogen. The electrodes are fabricated to be superhydrophilic (the contact angle of a water droplet on the surface is less than 5 degrees). The overpotential of the hydrogen evolution reaction (HER) is measured using a 3-electrode configuration with a structured electrode as the working electrode. The enhancement is attributed to several factors including increased surface area, increased wettability, and the impact of micro/nanostructures on the bubble formation and release. Special emphasis is placed on identifying and isolating the relative impacts of the various contributions.
Femtosecond laser surface processing (FLSP) is a powerful technique used to create self-organized microstructures with nanoscale features on metallic surfaces. By combining FLSP surface texturing with surface chemistry changes, either induced by the femtosecond laser during processing or introduced through post processing techniques, the wetting properties of metals can be altered. In this work, FLSP is demonstrated as a technique to create superhydrophobic surfaces on grade 2 titanium and 304 stainless steel that can retain an air film (plastron) between the surface and a surrounding liquid when completely submerged. It is shown that the plastron lifetime when submerged in distilled water or synthetic stomach acid is critically dependent on the specific degree of surface micro- and nano-roughness, which can be tuned by controlling various FLSP parameters. The longest plastron lifetime was on a 304 stainless steel sample that was submerged in distilled water and maintained a plastron for 41 days, the length of time of the study, with no signs of degradation. Also demonstrated for the first time is the precise control of pulse fluence and pulse count to produce three unique classes of surface micron/nano-structuring on titanium.
Femtosecond Laser Surface Processing (FLSP) is a powerful technique for the fabrication of self-organized multiscale
surface structures on metals that are critical for advanced control over energy transfer at a liquid/solid interface in
applications such as electrolysis. The efficiency of the hydrogen evolution reaction on stainless steel 316 electrodes in a
1 molar potassium hydroxide solution is used to analyze the role of surface geometry to facilitate the phase conversion of
the liquid to a gaseous state in the vicinity of the interface. It is found that the efficiency of the electrolysis process is
directly related to the separation of micro-scale features on an electrode surface. The enhancement is attributed to the
size of the valleys between microstructures controlling the contact between an evolving vapor bubble and the electrode
surface. The results suggest an alternative pathway for the tailoring of interfacial energy transfer on structured surfaces
separate from traditional benchmarks such as surface area and contact angle.
There are a growing number of unique self-organized micro/nanostructures created using femtosecond laser surface
processing that have been demonstrated. Although researchers have provided insight into the formation processes for
distinctive morphologies on specific materials, there is a need for a broader understanding of the physics behind the
formation of a wide range of morphologies and what parameters affect their formation. In this work, the formation
processes for mound structures on 316 stainless steel (SS) with growth above the original sample surface are studied.
The formation process for the structures on 316 SS is compared to similar structures formed on nickel using the same
technique. The structures are formed using 800 nm, 50 fs laser pulses, and are self-organized, meaning the structure
dimensions are much smaller than the spot size of the pulses used to create them. The formation dynamics were studied
using a stop-motion scanning electron microscope (SEM) technique, where the same location of an irradiated sample
was imaged in the SEM at various pulse counts. The result is a series of images showing the developmental progress
with increasing pulse counts. The structures form through a combination of fluid flow of the surface melt that results
after irradiation, preferential ablation of the center of the pits between structures, and material/nanoparticle redeposition.
We describe the evolution of laser damage spots on bulk nickel generated by multipulse femtosecond laser irradiation
with a 100 μm x 100 μm square flat-top beam profile as a function of the laser fluence and the number of pulses incident
on the target. This large-area irradiation simulates conditions associated with the interaction of femtosecond laser pulses
on a remote target. The larger area laser damage sites are characterized either by a series of self-organized surface
structures with micro- and nanoscale features or a deep circular pit rather than a crater that mirrors the beam profile.
Furthermore, the ablation rate of the deepest feature sharply increases above a laser fluence of 2 J/cm2; this increase is
associated with the creation of a deep circular ablation pit generated during ablation with the first few pulses on the
sample that continuously grows upon multipulse irradiation due to the focusing of incident laser energy into the pit by
the sloped pit surfaces.
Femtosecond laser direct writing (FLDW) has been widely employed to create volumetric structures in transparent
materials that are applicable as various photonic devices such as active and passive waveguides, couplers, gratings,
and diffractive optical elements (DOEs). The advantages of fabrication of volumetric DOEs using FLDW include
not only the ability to produce embedded 3D structures but also a simple fabrication scheme, ease of customization,
and a clean process. DOE fabrication techniques using FLDW are presented as well as the characterization of laserwritten
DOEs by various methods such as diffraction efficiency measurement. Fresnel zone plates were fabricated in
oxide glasses using various femtosecond laser systems in high and low repetition rate regimes. The diffraction
efficiency as functions of fabrication parameters was measured to investigate the dependence on the different
fabrication parameters such as repetition rate and laser dose. Furthermore, several integration schemes of DOE with
other photonic structures are demonstrated for compact photonic device fabrication.
In recent years, a major interest in surface as well as bulk property modification of semiconductors using laser irradiation
has developed. A.Kar et al. [1][2] and E.Mazur et al. [3] have shown introduction and control of dopants by long-pulse
laser irradiation and increased absorption due to femtosecond irradiation respectively. With the development of mid-IR
sources, a new avenue of irradiation can be established in a spectral region where the semiconductor material is highly
transparent to the laser radiation. The characterization of the light-matter-interaction in this regime is of major interest.
We will present a study on GaAs and its property changes due to pulsed laser irradiation ranging from the visible to the
mid-IR region of the spectrum. Long-pulse as well as ultra-short pulse radiation is used to modify the material.
Parameters such as ablation threshold, radiation penetration depth and thermal diffusion will be discussed.
The ability to integrate micro-channels for fluid transport with optical elements is attractive for the development of
compact and portable chip-based sensors. Femtosecond Laser Direct Writing (FLDW) in transparent materials is a
powerful tool for the fabrication of such integrated devices. We demonstrate the use of FLDW to fabricate coupled
micro-fluidic channels and optical waveguides towards an integrated sensing device for molecular detection.
Waveguides were directly written into the host material and channels were formed by modifying the molecular structure
through FLDW followed by wet chemical etching. Multiple host materials including chalcogenide glasses for IR
detection are discussed.
Optical coherence microscopy (OCM) is used to image femtosecond laser direct written buried structures created
within transparent media. Volumetric structures of optical damage and laser-induced refractive index change were
produced in fused silica and borosilicate glass, respectively. Noninvasive 3D imaging of the structures was
successfully demonstrated by a custom built OCM. High signal to noise ratio was obtained since the optical glasses
have high transparency at the probe wavelength centered at 800 nm.
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